Such a method is disclosed in U.S. Pat. No. 7,309,930 which relates to a vibration damping system. In the known method, the generator speed is measured and supplied to a speed regulator as an input quantity. The speed regulator determines a setpoint for the generator torque from the measured generator speed. In order to counteract vibrations of the system, the setpoint of the generator torque determined by the speed regulator is modulated. For this, a vibration damper generates an output signal which provides an additional torque for the generator. The output signal is calculated on the basis of the measured generator speed, whereby the frequency components of the generator speed signal which correspond to those vibration frequencies which are to be dampened flow in more intensely. The additional generator torque is to counteract the vibrations of the drive train as well as of the tower. In order to match the frequency behavior of the vibration damper to the wind turbine, the natural or resonant frequencies which are to be dampened are determined. For this, it is, inter alia, contemplated to evaluate signals of an acceleration sensor.
U.S. Pat. No. 7,400,055 discloses a similar method in which a sensor detects vibrations perpendicular to the rotor axis and an open-loop control influences the rotor in such a manner that the vibrations are dampened. The influencing of the rotor can be achieved via the generator torque or the blade pitch angle.
The common characteristic of the two known methods is that the occurring tower vibrations are to be counteracted by additional torque applied to the rotor. The methods rely on a correlation between the additionally applied torque and a counter torque resulting therefrom on the tower, more specifically, on the nacelle of the wind turbine. The exact correlation depends on the configuration of the wind turbine. Furthermore, external influences such as, for example, wind shear and oblique flows, which constantly change and are difficult to determine, play a major role.